A lithium ion secondary battery that contains a flammable nonaqueous electrolyte solution is equipped with a lot of members for safety measures, and as a result, an energy density per volume as a whole of the battery becomes low, which is problematic. In contrast, a lithium ion secondary battery that contains a nonflammable aqueous electrolyte solution does not need safety measures as described above, and thus has various advantages such as a high energy density per volume. However, a conventional aqueous electrolyte solution has a problem of a narrow potential window, which restricts active materials etc. that can be used.
As one means for solving the above described problem that the aqueous electrolyte solution has, Non Patent Literature 1 discloses that dissolving a high concentration of lithium bis(trifluoromethanesulfonyl)imide (hereinafter may be referred to as “LiTFSI”) in an aqueous electrolyte solution can expand the range of a potential window of the aqueous electrolyte solution to 1.9-4.9 V (vs. Li/Li+). In Non Patent Literature 1, such an aqueous electrolyte solution of a high concentration, LiMn2O4 as the cathode active material, and Mo6S8 as the anode active material are combined, to form an aqueous lithium ion secondary battery.
Non Patent Literature 2 discloses that combining an aqueous electrolyte solution of a high concentration with C/TiO2 to form SEI (Solid Electrolyte Interphase) can result in an expanding potential window of the aqueous electrolyte solution on the reduction side to 1.83 V (vs. Li/Li+).